The present invention relates generally to transformers for powering luminous loads and more particularly, this invention pertains to secondary ground fault protection for neon tube transformers.
For luminous tube transformers presently utilized in industry, the output voltage from one output terminal to ground cannot exceed 7500V. To provide a design capable of producing output voltages in excess of 7500V, a xe2x80x9cmid-point groundedxe2x80x9d secondary design is employed in which two secondary coils are used. These coils produce voltages that are 180xc2x0 out of phase with each other in order to develop a terminal-to-terminal voltage that is twice that measured from any one terminal to ground.
New industry regulations have been developed that require the addition of secondary ground fault protection to such designs. As noted by UL 2161 subsection 20.4 xe2x80x9cAn isolated output neon supply shall have a current to ground that is 2 milliamps or less when measured in accordance, with the Isolated Output Determination Test, Section 24A.xe2x80x9d (revised Mar. 16, 1999). Subsection 24A.1 then notes: xe2x80x9cTo determine compliance with 20.4, a neon supply is to have any protective circuitry that prevents the supply from operating without an output load connected to it disabled. The neon supply is to be connected to a source of supply adjusted to rated input with no load connected to the output. While energized, the current from each output lead or terminal to ground is to he measured. The maximum current shall not exceed 2 mA rms.xe2x80x9d (added Mar. 16, 1999). The intent is to provide a level of protection and to detect i secondary side fault to ground as a measure to reduce any potential fire hazards that may exist as a result of arcing.
As shown in FIGS. 1 and 2, mid-point grounded transformer designs 100, 200 for prior art applications are typically constructed with input terminal means 130 for receiving an input source of power, a primary winding 103 also known as a primary coil 103 with input leads 134, a core 106, at least one secondary winding 104 also known as a secondary coil 104 with output leads 136, high voltage external output terminals 132, external ground terminal 112, and chassis 1108. One endpoint 102 of each secondary coil 104 is electrically common to form a secondary midpoint 110. This secondary midpoint 110 in turn is connected to the transformer core 106. The core 106 is then connected to earth ground 114. The ends 102,1202 of the secondary coil 104 and earth ground 114 are also connected to the transformer enclosure 108, 208, if the enclosure is conductive, by a ground lead 138 providing a chassis 108 ground connection to earth ground 114. A ground wiring terminal 112 is provided on the enclosure 108 that provides a direct connection to the secondary midpoint 110 and the earth ground 114.
The luminous tube loads 116 are operated by the transformer designs 100, 200 using wiring connections 118, 218 shown in FIG. 1 and FIG. 2. FIG. 1 illustrates a xe2x80x9cseriesxe2x80x9d connection 118 of the luminous tube load 116. A possible problem with this method is that the length of conductor 120, shown is high voltage potential wiring 122, required to connect the secondary windings 104 to the tube load 116 may become excessive causing higher than acceptable leakage currents. This problem is overcome by utilizing a parallel wiring connection 218 shown in FIG. 2. in which the length of high voltage wiring 122 is minimized. Longer wiring runs are limited to the grounded conductor 124. This parallel type of wiring 218 of the luminous tube load 116 is commonly referred to as xe2x80x9cMid-Point Groundedxe2x80x9d 218. More recent nomenclatures may also refer to this as a xe2x80x9cMid Point Wiredxe2x80x9d 218 tube load.
FIG. 3 of the drawings shows a prior design using a grounded series connected protected circuit 300. With the addition of Secondary Ground Fault Protection 302 connected between the midpoint 110 and the ground 114, the fault path 304 now passes through a sensor, shown as Secondary Ground Fault Protection 302, before connecting to ground 114. When a series tube connection 118 is employed as shown in FIG. 3, a secondary fault is detected by the Secondary Ground Fault Protection 302 by sensing the current flow on the fault path 304 from ground 114 to the coil mid-point 102.
As shown in FIG. 4, if the tubing load 116 is connected using a Mid-Point Wired parallel connection 218, the luminous tube load 116 current paths 402 are the same as a ground fault current fault path 304. With this connection, any imbalance between the current flowing from S2-to-ground and S1-to-ground, will appear as a ground fault. This would result in nuisance tripping of the Fault Protector 302.
Similarly, as shown in the series connection 118 of FIG. 5, if the high voltage transformer to tube load wiring 122 exhibits a significant amount of capacitively coupled leakage current, shown schematically as the capacitor 502, such current will appear as a ground fault. This too would result in nuisance tripping of the fault protector 302.
Finally, industry requirements dictate that a ground fault protected transformer either: (a) detect faults while chassis ground 112 is not connected to earth ground 114; or (b) shutdown transformer operation if no earth ground 114 connection is present.
In field applications, the ability to provide a reliable, low impedance earth ground 114 connection may be limited as a result of remote installation such as rooftop or pole mounted installations. The resultant high-impedance or xe2x80x98noisyxe2x80x99 ground connection can result in nuisance tripping of the fault circuit 302.
As an alternative to such protection, the transformer may utilize an isolated secondary coil design in which the output voltage does not have a measurable fixed reference to ground. A transformer or power supply of isolated design is considered to inherently provide Secondary Ground Fault Protection since there is no tendency for a xe2x80x9cfloatingxe2x80x9d voltage to seek ground. Such isolated designs are subject to fault tests in which one output is grounded. In such a fault test, the ungrounded output cannot produce a voltage in excess of 7500V. If the output does produce an output in excess of 7500V, to ground, the addition of secondary ground fault protection circuitry is required. The present invention provides an apparatus and method for providing this protection with series or mid-point wired loads. What is needed, then, is an apparatus for improved detection of fault currents in a luminous tube transformer circuit with educed false tripping. This improvement is provided by the Secondary Ground Fault Protected Neon Transformer described herein.
The present invention is designed to provide a novel transformer utilizing an isolated secondary winding design and incorporating a secondary ground fault protection circuit to provide the end user with the option of series or mid-point wired tube loads. Such a design has been proven to provide a reduction of nuisance tripping of the fault circuit as a result of capacitive coupling of output wiring, unbalanced luminous tube loads, or lamp arc transients.
The apparatus of the present invention is a transformer assembly for powering a load with a Secondary Ground Fault Protection circuit for an isolated secondary. The fault path is isolated from ground and the return terminal is isolated from the secondary midpoint for series and mid-point load connection schemes, including schemes using the midpoint as a ground connection. As an exemplary use of this isolation, a power control system is connected between the primary winding and the input terminal with the ground fault detection circuit connected in the fault path. The ground fault detection circuit is operable to detect a fault and activate the power control system to disconnect the source of power from the primary winding in response to detecting the fault.
Also disclosed is a high frequency filter adapted to reduce the effects of high frequency transients. A further aspect teaches a capacitive reactance connected between the input terminals and the ground terminal, so that the capacitive reactance an provide a ground fault path for fault signals. Yet a further improvement teaches he improved performance of an optocoupler using a breakover device for improved bias control.
Objects of the present invention include: 1) a high voltage isolated virtual midpoint return terminal, 2) integration of an isolated secondary transformer with a ground fault detection circuit; 3) integration of an isolated secondary transformer with a ground fault detection circuit while maintaining secondary isolation; 4) use of a capacitive component between line voltage supply and chassis ground to provide alternate ground fault path for fault signals; 5) use of a high frequency filter to reduce erroneous ground fault detection of transient events; 6) use of high impedance between transformer secondary windings and chassis ground to maintain isolation effect; 7) use of diac/breakover component to desensitize optocoupler performance: and 8) use of a transistor to discharge ground fault sensor filter capacitors.